1. Field of the Invention
The present invention relates to a recording apparatus utilizing an electrothermal converting element, and more particularly to an ink jet recording apparatus provided with a temperature detecting-correcting circuit for detecting the heat generated by the electrothermal converting element as temperature. The present invention also relates to an ink jet recording apparatus for effecting recording by discharging ink, utilizing the electrothermal converting element.
2. Related Background Art
FIG. 8 is a schematic circuit diagram of a conventional ink jet recording element substrate, wherein shown are an ink jet recording element substrate 501, a temperature detecting diode 502 for detecting the heat generated by a heater as temperature, electrothermal converting elements (heaters) 504 for generating thermal energy, switches 505 for determining the timing for current supply to the heaters, a power supply line 506 for applying a predetermined voltage to the heaters, thereby supplying current thereto, power transistor units 508 for supplying the heaters with desired currents, an ink discharge data unit 509 for transferring and storing, for each heater, data whether or not to supply each heater with the current for ink discharge, and a pulse width calculation unit 510 for measuring the voltage of the temperature detecting diode 502, converting the measured voltage into temperature and calculating the turn-on time of the switches 505. There is already known such substrate on which the electrothermal converting elements (heaters), the driving circuits therefor and the temperature detecting elements are integrally formed.
FIG. 9 is a chart showing pulses of different widths for turning on the switch 505 so as to obtain a substantially same ink discharge amount at different temperatures T, FIG. 10 is a chart indicating the conversion of the output voltage of an ideal temperature detecting diode 502, whose characteristics are known, into the temperature, and FIG. 11 is a chart for calculating the pulse width for turning on the switch 505, based on the temperature converted from the voltage. In these charts, 701, 702 and 703 are points indicating the temperatures at different voltages, and 801, 802 and 803 are points indicating the pulse widths at different voltages.
FIG. 12 is a circuit diagram showing the details of the function of the temperature detecting diode and the pulse width calculation unit shown in FIG. 8, wherein shown are an ink jet recording element substrate 901, and a temperature detecting diode 902. In this example there are employed two temperature detecting diodes. There are also shown a resistance 903 of an aluminum wiring between the temperature detecting diode and an external electric contact (pad), a heater (electrothermal converting element) 904, a switch 905, a power supply line 906, an external electrical contact (pad) 907, a pulse width calculation unit 910, an absolute temperature detecting unit 911, an arithmetic operation unit 912 for reading the data of the absolute temperature detecting unit 911 and of a monitor 915 and outputting a pulse width, a constant current source 914 for supplying the temperature detecting diodes 902 with a constant current I.sub.F, and a monitor 915 for measuring a voltage 2V.sub.F corresponding to the two diodes 902 when the current I.sub.F is supplied thereto from the constant current source 914.
Data of a number corresponding to that of the heaters 504 (904) and the power transistor units 508 are supplied to and stored in the ink discharge data unit 509 shown in FIG. 5. By turning on the switch 505 (905) for an appropriate period, a current is supplied to the power transistor unit 508 and the heater 504 through the power supply line 506, according to such period. If the turn-on time of the heater 504 is so set as to obtain a desired ink dot diameter in an initial state where the temperature is stable and the current supply is repeated with such a turn-on period, the heat generated by the heater 504 is transmitted to the ink through the element substrate with the lapse of time, whereby the viscosity of ink varies, inducing a variation in the ink discharge characteristics. Therefore, if the heater is turned on with the same time as in the initial state, the ink discharge amount increases to form a larger dot at the ink landing spot, whereby the image density becomes uneven.
The temperature of the element substrate is detected in order to avoid such drawback. In the following there will be explained the method of temperature detection, with the two diodes shown in FIG. 12. In a state where the heater 904 is not activated and the ink jet recording element substrate 901 and the pulse width calculation unit 910 are in a stable state of the same temperature, the absolute temperature T.sub.O is measured by the absolute temperature detection unit 911. Also a constant bias current I.sub.F is supplied by the constant current source 914 to the temperature detecting diodes 902, and the voltage 2V.sub.FO thereof is measured by the monitor 915. In this manner the diode voltage 2V.sub.FO at the temperature T.sub.O .degree. C. can be obtained as a number value, and, even in the presence of a temperature increase in the ink jet recording element substrate 901, the temperature T thereof can be estimated from a temperature coefficient 2.multidot..differential.V.sub.F /.differential.T, 2V.sub.FO, 2V.sub.F and T.sub.O according to the following equation: EQU T=T.sub.O +(2V.sub.F -2V.sub.FO)/(2.multidot..differential.V.sub.F /.differential.T) (A)
Based on the thus calculated temperature T, the desired pulse width corresponding to the desired ink discharge amount is determined from the curve in FIG. 11. It is therefore rendered possible, even when the temperature of the ink jet recording element substrate 901 rises, to obtain a constant ink discharge amount, thereby obtaining the ink dots of a constant diameter, by turning on the switch 905 with such a pulse width.
In the following a more detailed explanation will be given with reference to FIGS. 8 to 11. It is assumed that the reference temperature in the initial state is T=25.degree. C. as shown in FIG. 9, and that the switch 505 is given a turn-on time (pulse width) t.sub.2 corresponding to such a reference temperature. Repetition of the heat generation under such conditions increases the temperature of the ink jet recording element substrate 501 by the heat generated by the heater 504, whereby the temperature of the ink also rises. The pulse width calculation unit 510 (910) reads the output 2V.sub.F of the temperature detecting diodes 502 (902) and obtains the temperature based on the temperature characteristic curve based on 2V.sub.FO and T.sub.O as shown in FIG. 10 and the obtained output 2V.sub.F. Thus, the temperature T is identified as 70.degree. C. if 2V.sub.F =2V.sub.F(70). FIG. 11 shows a characteristic curve indicating the correspondence between the pulse width and the detected temperature, for obtaining a certain ink discharge amount. The pulse width for obtaining, at 70.degree. C., an ink discharge amount the same as that at 25.degree. C. is determined from FIG. 11, and is identified as t.sub.3 (narrower than t.sub.2 by .DELTA.t.sub.2) indicated by a point 803 corresponding to 70.degree. C. Then the switch 505 is given a signal of a pulse width t.sub.3, whereby the same landed ink spot can be obtained even in the elevated temperature state of the ink. Also in case the external temperature drops and is identified as T=-10.degree. C., a pulse width t.sub.1 indicated by a point 801 is similarly selected from FIG. 11. The characteristic curve shown in FIG. 11 defines the condition for obtaining a constant ink discharge amount (ink amount at the ink landing point), and this characteristic curve remains the same both in the conventional configuration and in the embodiments of the present invention.
FIG. 13 is a detailed block diagram of the pulse width calculation unit 901 shown in FIG. 12, wherein shown are an absolute temperature detecting unit 1011, an arithmetic operation unit 1012, a constant current source 1014, a monitor 1015 for detecting V.sub.F, an amplifier 1021, an A/D (analog-to-digital) converter 1022, a CPU 1024, another A/D converter 1025, an operation program 1026, a bias resistor 1027, and a temperature-dependent variable resistor 1028.
The temperature of the substrate 901 can be detected according to the foregoing equation (A), from the temperature T obtained in the absolute temperature detection unit 1011 and the voltage V.sub.F of the diode 902 of the substrate 901 obtained by the monitor 1015. The substrate temperature thus obtained can be converted into the desired pulse width for providing the constant ink discharge amount, based on the pulse width-temperature characteristics shown in FIG. 11.
However, the conventional technology explained above has been associated with the following drawbacks:
1. In the foregoing description of the conventional technology, the temperature detecting diodes 502 provided on each substrate or head are assumed to have the ideal characteristics as shown in FIG. 10 and to be free from any fluctuation in the characteristics. In fact, they show a certain fluctuation in the characteristics in the manufacturing process, but the extent of such fluctuation cannot be measured since such measurement has to be made by giving a temperature change to each substrate or head. For this reason, the characteristics have been estimated from the experience in the past. PA1 2. Such fluctuation in the temperature characteristics provides a detected temperature higher or lower than the actual temperature. The pulse width for obtaining the predetermined ink discharge amount is determined, according to the pulse width-temperature curve shown in FIG. 11, by such detected temperature, the turn-on time of the switch 505 (905) involves an error whereby the ink discharge amount shows a considerable variation to result in a change in the size of the landed ink dot. PA1 3. The heater 504 (904) may be given a current for an unnecessarily long period for the above-mentioned reason, so that the service life of the heater may be shortened in comparison with the case where the current is given only for the desired period.